Only way to fix the clay problem is bring in compost + topsoil mix. But you don't have to do it all at once either. A 50/50 mix of good soil with the NC clay still can grow dahlias just fine. However you may lose a few tubers with spring rains so if there is one u must keep, pot it up, take a cutting then plant out the potted dahlia it will catch up to the others.
I think there is an old saying of $50 hole for a $10 plant. Good soil makes it a lot easier to grow them and since we are not the PNW its much harder to grow with clay + 90% humidity summers 😂
You can also put down Pelletized Gypsum each fall or spring it will help break up the clay. I put down a bag every few years. 40 pound bag at home depot is $16 I got some last week.
Here is a pretty good article on soil from the local AG School. And you can also mail in soil samples for free to NC State they test all of it for free.
https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients
